Where do gearbox cases crack?

We all hear of plastic and nylon gearboxes ‘blowing up’ or breaking… getting busted or whatever… obviously if a plastic gear breaks, there is bits of gears in the bottom of the gearbox or just a gear in 2 chunks, nylon gears probably strip teeth rather than break… what I am interested in, is where do the stronger nylon gearbox casings generally break/fail?

Do they crack/split completely at some particular point? Do they develop hairline cracks around the bush/bearing holes?.. like, is there common places they all fail? Or do they crack top to bottom or horizontal on the sides?

Does anyone have any pics of broken cases or failed gearboxes? I am just wondering where the weaknesses are in the cases? Whether they fail, they all seem to go in one or 2 particular places… or they just crack/fail anywhere at all?

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Yes, this would be a great resource to have. Pictures of things that have failed.
Putting some extra support around the external part of the gearbox in some of these areas could prevent it in the first place.
That support could be physically on the gearbox or on the blaster pushing up against the gearbox.

But first we need the data to determine where.

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That all depends on which way I’m holding the gearbox as it’s being thrown ( yes I’ve done it couple times) in frustration. Like when started modding my firstst GB (JM scar V2) for the reverse latch gave me slot of grief.

From my experience it’s been where the cyclinder head and t piece meet.

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APS gearbox cracked,wasn’t even pushing it hard.
Don’t have a picture but I highlighted the weak spot in the picture.

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Wow. Just goes to show, metal isn’t always the answer.
That’s die-cast metal, not milled isn’t it?

Problem with cast metal is that it does not bend or flex. I learnt that with the old matchbox toy cars when I was a kid :sob:

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Yeah, my next box will be retroarms or the mk one that’s Aussie made.


yeah, there is alot of stress going through the gearbox cases… I am not an engineer, but having built gyrocopters, I sort of know how metal and fatigue can slowly work at things and cause cracks from any weak spot… like holes, rough edges and welding.

From what I have read and researched, I am thinking that probably a good nylon case set is the most likely to survive the rigors, yes, they may eventually crack or break… but they only cost $10 bux from China to replace. Maybe weak spots can be strengthened by arolditing some fiberglass cloth over certain areas… smoothing rough edges… smoothing holes before using.

For me, I would rather not spend up to $200 on metal gearbox cases… which unless they are made of single crystal titanium, may also crack, cast alloy, and that cast ‘muck metal’ like the dinky toys… all have large porous grains in them when looked at under a magnifying system, so prone to cracking… CnC.d from a billet would obviously be best… but the cost?

I think I will just buy a bunch of nylon cases and replace them as needed… hopefully, from this thread, I may be able to reinforce notoriously weak spots… and look for the main causes of the cracks beginning.

One thing I think may be one cause of cracking, is the final gear that runs the ladder retracting the plunger… in nylon (or plastic) the gear is a little out of balance and carries little mass to cause harmful vibration… but when a metal gear is used, the out of balance is probably quite large… causing large forces in the box… forces the cases have to deal with…

I am thinking that balancing this gear may give the cases an easier time? My metal gear sets are due any time, I will be checking this gear for balance and maybe trying to balance it, to try and get nylon cases to last longer.

Has anyone tried balancing any of the metal gears?:thinking:

I’ve only ever used one nylon box and was put off by the flexing. I wasn’t keen to put anything like a titan in one just in case. I wonder if there prone to warp with super high speed set ups?

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I think so… the higher they spin… the greater the forces involved. Also, there is multiple forces at play… the whirring of the gears cause all sorts of harmonic vibrations for one… then the ramming backwards and forwards of the plunger… I am guessing, at some stage, the force from the rotating gears and the forces from the plunger snapping back and forward could meet and create quite large spikes… which would, in turn, work on weak points and imperfections…

Thats the sector gear your referencing bro which pulls the piston back. When you say balance gears are you referring to shimming the internal gears so they mesh perfect with minimal resistance.
Aside from a couple of the metal boxes that are poor in quality compared to the others I’ve found alot of the time the fractures or breaks due to stress have been from incorrect care of the gearbox, from incorrect shimming, lack of lube :stuck_out_tongue_closed_eyes:, also no regular servicing/not polishing and cleaning up the shit that builds up in the shell over time. It should be treated the same way as you would with your motorbike or car. Regular upkeep and maintenance for its lifetime.

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It’s comes down to the quality of the build. In one of my blasters I’ve got a singularity nylon box (gen8) it’s worked off its tits 12;1 gears, 1.4 unequal, alloy piston to name a couple of the mods. 310-330 fps and it loves the CQB

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The nylon or plastic gears out of my Gen9 weigh bugger all, so balance probably isn’t an issue… I am interested when my metal gears get here, if that last gear that runs the plunger is as out of balance as it looks… maybe it has weight added to compensate for the teeth missing from the ‘release’ side.

If that particular gear is very out of balance, spinning at high speed when working would provide plenty of shaking… which would stress everything.

Of course, it is going to make an impact as it picks up the plunger then lets it go… I find these things fascinating :grin:


By dressing drilled holes, by running a bigger drill backwards to smooth small (microscopic) cracks… can stop cracks which can become major issues at a later date. Tiny cracks can grow very quickly when vibrations and stress occur

The gears don’t spin very fast at all.
If you are pushing 20rps the gears are doing 20rps (rev per sec)and for part of that its engaged with the piston rack.
But you can get a balanced sector gear if you want.

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I would have thought that they would spin quite fast? The motors I have are 37000 rpm… so that means the pinion is doing 37000 rpm? That must mean the gear driven by the pinion is going to be rotating rather briskly… obviously the other 2 gears slow down as per design… to get the plunger to operate at the appropriate speed…

I wouldn’t like to service mine like my car and motorbike… I service them by WIRE… (When I Remember Eh) and flog them sensless in between :sweat_smile:

37000rpm is the no load speed, you won’t get anywhere near that in real use.
And at 18-1 it’s one 18th of that

Servicing is a very important part of owning a gel blaster.
I oil all the bearings and check screws etc.weekly and clean /re grease every month,check clearences etc. and check the motor brushes.
If you don’t performance will suffer and you will be buying new parts regularly.

my servicing is just: Pick it up, flush it out with some new, clean balls… then:

Repeat :sweat_smile:

Sounds like 98% of the gel blaster owners in Australia :roll_eyes:


Yes… hammer it until it dies… then throw more money at it…

works for me :+1:


Actually it’s not, that’s why your gun is down waiting for parts lmao